1. Field of the Invention
The present invention relates to a tilt control method and optical disc apparatus applying near-field light to perform at least one of recording and playing back of a signal.
2. Description of Related Art
In the related art, optical disc apparatuses applying laser light in order to increase recording density on an optical disc are proposed for recording or playing back (reproducing) signals using near-field light. With optical disc apparatuses employing near-field light, it is necessary to (gap servo) control a gap between disc and an end surface of an object lens etc., such as an SIL (Solid Immersion Lens) arranged in an optical head to be a distance (near field) at which near-field light is generated.
A method exists for controlling a gap based on an amount of returning laser light reflected from a disc side. In the case of laser light of a wavelength of, for example, 400 nm, a near field state is typically entered at half a wavelength or less. As a result, when the gap is a distance of 200 nm or more, i.e. in a far-field state, light from a laser light source incident on an end surface of an SIL at an angle causing total reflection is reflected completely at the end surface of the SIL and the amount of returning light is therefore constant. When the gap length is a distance of 200 nm or less, i.e. when a near field state is entered, the amount of returning light becomes smaller because part of the light incident on the end surface of the SIL at an angle causing total reflection passes through the end surface of the SIL. When the gap between the SIL and the disc is zero, i.e. when the SIL and disc make contact, the entire light incident on the end surface of the SIL at an angle causing total reflection passes through the end surface of the SIL and the amount of returning light is zero. With this technology, an amount of returning light is detected by a photodetector. A gap servo operation is then carried out for the SIL taking this detected amount of returning light as feedback to an actuator (for example, a biaxial device for carrying out focusing servo and tracking servo operations) of the SIL. For example, refer to Japanese Patent Laid-open Publication No. 2002-319157 (paragraph [0025], FIG. 7.